What is Content Testing? Are there content testing tools available, or is content simply tested manually for quality?
"Content testing" typically refers to the practice of testing whether or not your content is suitable for the audience in question, and whether or not they can understand and comprehend it. I believe it is more geared to internet-based content, but content testing is applicable to marketing, advertising, non-fiction, and probably even fiction - just about any discipline that relies on writing for a particular audience.
For good content testing, you need to know who your audience is, for example, literacy level, and any interests you may feel are relevant in relation to the content you've written. You're not really interested in whether or not the reader is going to like what you've written, but more whether or not they're going to actually understand what you've written.
Understanding can be greatly influenced by word choice, length of sentences, grammar, and sentence structure, so content testing would pay close attention to the way the author is trying to express him or herself, and whether it's done concisely and simply. There are some automated tools out there that attempt to give you an idea as to the "readability" of your content (for example, Readability Score), but they are not as good as relying on readers to give feedback by means of a test.
For webpage content, you would refer to this as a usability test; you could instruct an individual to "Buy the cheapest jersey that you like", for example, and see if they can successfully use the site's content to carry out the instruction. There are other techniques like Cloze tests.
If you were testing a non-fiction book, you could carry out similar sorts of tests, asking questions related to the content of what they just read in order to gauge their comprehension of the subject matter, or plot if it's a fictional book.